On the path toward fertilization, some of the changes in a human sperm occur within the male body. Millions of sperm are propelled through the vas deferens into the ampulla, where they are combined with are seminal vesicle secretions, resulting in seminal fluid. Additional secretions from the prostate gland and bulbourethral glands are further combined into the fluid. Usually through ejaculation directly into a female partner, the seminal fluid reaches the vagina, where it encounters cervical mucus that has thinned because of ovulation. The combined fluid facilitates the sperm’s movement into the fallopian tubes, or oviducts.

The capacitation process involves the motility of the sperm tails, or flagella, and enzyme production; both are necessary to help sperm penetrate the ovum’s membranes in the acrosomal reaction.

In regard to the ovum, within the female body, it has been produced as an oocyte in an ovary and then moves into the oviduct. The ovum release chemical attractants that draw the sperm. Each ovum has two layers of membrane: the outer one is the corona radiata, and the inner one, the zona pellucida. The sperm must get through both. After a single sperm has done so, the ovum generally repels further sperm in the blocking process.